The field of the invention is that of wide passband antennas with hemispherical or quasi-hemispherical radiation patterns. More specifically, the invention relates to resonant helix antennas working in two neighbouring frequency bands that correspond to transmission and reception, and especially to the decoupling of these two channels, and hence to the duplexer function.
The antenna of the invention can find application especially in mobile satellite communications between users in fixed positions and moving bodies of all kinds for example, aeronautical, maritime or land-based bodies. In this field, several satellite communication systems are being implemented or are currently being developed (these include, for example, the INMARSAT, INMARSAT-M, GLOBALSTAR, and other systems). These antennas are also valuable in the deployment of personal communications systems (PCS) using geostationary satellites.
For all these systems, which provide for links with geostationary satellites, the very great difference in incidence between the signals received or transmitted requires that the antenna should have a radiation pattern with hemispherical coverage. Furthermore, the polarisation has to be circular with a ratio of ellipticity of more than 5 dB in the useful band.
More generally, the invention can be applied in all systems requiring the use of a wide band, a radiation pattern with hemispherical coverage, circular polarisation and a good ratio of ellipticity.
In the above-mentioned fields of application, the antennas must have the above-mentioned characteristics either in a very wide passband in the range of 10% or in two neighbouring sub-bands respectively corresponding to reception and transmission.
The patent FR-89 14952 filed on behalf of the present Applicant has already described a known type of antenna well suited to such applications.
This antenna, called a resonant quadrifilar helix (RQH) antenna has characteristics very close to the criteria laid down in a frequency band generally limited to 5% owing to problems of impedance matching. Operation on two bands is possible by using dual-layer RQH antennas. These antennas are formed by the concentric xe2x80x9cnestingxe2x80x9d of two electromagnetically coupled coaxial resonant quadrifilar helices.
A quadrifilar antenna is formed by four radiating strands. An exemplary embodiment is described in detail in A. Sharaiha and C. Terret, xe2x80x9cAnalysis of quadrifilar resonant helical antenna for mobile communicationsxe2x80x9d, IEEExe2x80x94Proceedings H, Vol. 140, No. 4, August 1993.
In this structure, the radiating strands are imprinted on a thin dielectric substrate and then wound on a cylindrical medium that is radioelectrically transparent. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end with conductive segments positioned on the base of the lower part of the supporting cylinder. The four strands of the helix are therefore excited through these conductive segments.
This antenna conventionally requires a supply circuit that excites the different antenna strands by signals having the same amplitude in phase quadrature. There are several known techniques used to obtain a supply circuit of this kind.
In the above-mentioned document xe2x80x9cAnalysis of quadrifilar resonant helical antenna for mobile communicationsxe2x80x9d, this function is fulfilled by means of a structure using couplers (3 dB, xe2x88x9290xc2x0) and a hybrid ring. This assembly is implanted on a printed circuit placed at the base of the antenna.
This technique has the advantage of being relatively simple to make and implement. By contrast, it leads to a non-negligible space requirement as compared with the antenna (which for example may have a size of about ten centimeters). This drawback makes this approach incompatible with many applications, especially when maximum miniaturisation is required.
According to a second technique described in J. L. Wong and H. E. King, xe2x80x9cUHF satellite array nulls adjacent signalsxe2x80x9d (Microwaves and RF, March 1984), each bifilar helix may be supplied by a xe2x80x9cfolded balunxe2x80x9d type of coaxial symmetrizer. The two bifilars helices are then excited in phase quadrature by means of a hybrid coupler.
The advantage of this method is that it requires the use of only one external hybrid element. By contrast, the symmetrizer/adapter assembly used for this type of antenna (made for example out of a coaxial section whose core and sheath form a dipole) is complex and bulky.
Furthermore, this type of assembly has the drawback of forming a sort of passband filter with a band that is still excessively narrow.
A third, more complex technique is described in C. C. Kilgus, xe2x80x9cResonant quadrifilar helixxe2x80x9d (Microwave Journal, December 1970). The coaxial supply line is split at its end to form a symmetrizer. The phase quadrature is provided by adjusting the length of the strand.
This technique is used to eliminate hybrid couplers. However, it has the drawback of requiring a delicate adjustment of the length of the strand. Furthermore, the antenna is no longer symmetrical and the structure will be more complex. Besides, this method remains specifically reserved for systems using a narrow working band.
In the case of two-way antennas having to carry out the transmission and the reception of signals, it is naturally necessary, as far as possible, to decouple the transmission frequency band and the reception frequency band, which are generally close to each other.
This is the role of the duplexer which is generally placed at the supply point of the antenna. There are several known types of duplexer. Gord Neilson and John Mchory, xe2x80x9cRF filters and diplexers for cellular applicationsxe2x80x9d (Antem ""90) describes several types of duplexers used in the field of radiocommunications.
In general, these known devices have the drawback of taking the form of an element that is independent and complementary to the antenna. They therefore entail considerable space requirements, especially when the antennas are very small-sized.
Furthermore, these are elements that are complicated to make and implement. Consequently, their cost price is great as compared with the cost of the antenna itself.
Finally, these duplexers act as filters and may therefore introduce losses of useful parts of signals.
The invention is aimed in particular at overcoming these different drawbacks of the prior art.
More specifically, an aim of the invention is to provide an antenna and its system of supply (hereinafter the term xe2x80x9cantennaxe2x80x9d includes the antenna proper as well as its supply system) having two sub-bands that are sufficiently decoupled not to require the presence of an additional standard duplexer.
In other words, the invention is aimed at providing a two-way antenna that fulfils the duplexing function in a simple and efficient manner without using known duplexers.
Another aim of the invention is to provide an antenna of this kind that has a low cost price and can easily be made on an industrial scale. In particular, the invention is aimed at providing an antenna of this kind that can be manufactured in a very small number of successive operations.
Another aim of the invention is to provide an antenna of this kind that does not require specific and complex setting operations.
Yet another aim of the invention is to provide an antenna of this kind (and especially the supply system of such an antenna) taking up little space as compared with known devices.
The invention is also aimed at providing an antenna of this kind that achieves an equal-amplitude excitation of the four strands and a precise phase quadrature relationship and hence high quality of circular polarisation in both sub-bands.
These aims as well as others that shall appear hereinafter are achieved according to the invention by means of a helix antenna with integrated duplexing means comprising two decoupled coaxial helices, each formed by radiating strands printed on a substrate, each of said helices being associated with an independent and miniaturised structure for the wideband supply of said radiating strands, said supply structures being printed on said corresponding substrate and comprising at least one hybrid coupler made out of semi-localised (or xe2x80x9cnon uniformly spacedxe2x80x9d) elements so as to reduce the dimensions thereof.
The making of the antenna strands and of the supply structure in the form of printed elements enables the production of the antenna, its supply structure and the duplexer in one and the same operation without any specific connection means and in a particularly small format.
The use of hybrid couplers made out of semi-localised elements can be used to obtain the set of desired qualities, and especially to reduce the space requirement of the assembly as compared with conventionally used lines.
Since the two layers forming each of said coaxial helices are perfectly decoupled, this structure directly fulfils the role of a duplexer without any additional element. The supply points of each of the helices respectively and directly correspond to the transmission signal and to the reception signal.
Thus, a very simple low cost two-way antenna is obtained.
Advantageously, said helices, when they are laid out flat, have strands with directions that are symmetrical to the axis of said antenna and are wound in opposite directions of winding so that said strands are substantially parallel.
This technique enables the printed face of the internal helix to be pointed inwards and that of the external helix to be pointed outwards.
Preferably, in order to decrease the decoupling, the points of excitation of said quadrifilar helices are offset with respect to each other in a plane perpendicular to the axis of said helices. According to one advantageous embodiment, they are offset by 135xc2x0.
The invention can be applied to all types of helix antennas. According to a preferred embodiment, said helix is a quadrifilar helix, formed by four radiating strands supplied by a supply structure comprising three hybrid couplers.
Advantageously, in the last-named case, said supply structure comprises a first 180xc2x0 hybrid coupler associating a supply input and/or output of said antenna with two intermediate outputs and/or inputs phase-shifted by 180xc2x0 and two 90xc2x0 hybrid couplers each associating one of said intermediate outputs and/or inputs of said first hybrid coupler with one of the ends of two of said radiating strands.
According to an advantageous embodiment of the invention, said antenna is mounted on a support having a first part and a second part that are distinct with different values of permittivity, said first part bearing said radiating strands and said second part bearing said supply structure.
Preferably, said first part bearing the antenna strands has a permittivity greater than 1.
It is thus possible to further reduce the amount of space taken up by the antenna.
An antenna of this kind as described here above may be used alone or in an array of antennas.
The invention also relates to the manufacture of said antennas. This manufacture is particularly simplified as compared with the prior art techniques.
According to a first method of manufacture of a resonant helix antenna, the following steps are planned:
the printing, on a plane substrate, of at least two radiating antennas designed to form a helix and of an independent, miniaturised structure for the wideband supply of said radiating strands comprising at least one hybrid coupler made out of semi-localised elements so as to reduce the dimensions thereof;
the winding of said substrate around a cylindrical support.
According to a second method of manufacture of a resonant helix antenna that is even more simple to implement, the following steps are performed:
the obtaining of a cylindrical support bearing a substrate;
the printing, on said substrate, of at least two radiating antennas designed to form a helix and an independent, miniaturised structure for the wideband supply of said radiating strands comprising at least one hybrid coupler made out of semi-localised elements so as to reduce the dimensions thereof.